Fibre-channel, arbitrated-loop (FC-AL) storage systems generally require external FC cabling to interconnect multiple storage enclosures together. Such cabling requires different cable lengths depending on how close or far the enclosures are located with respect to each other. As serial interfaces operate at higher and higher speeds (e.g. 3 Gb/s, 6 Gb/s 10 Gb/s), it is important that high speed signal tuning be performed when such high speed signals are routed through cables in order to improve signal transmission. Given the practical requirement for different cable lengths, it becomes important to selectively tune the high speed signal characteristics as a function of the cable length. Thus, it is common practice to amplify signals that traverse long cables and less-amplify signals that traverse shorter cables. In order to determine the signal amplification (e.g. pre-emphasis or de-emphasis), the cable length must be known.
In FC-AL cabled storage systems a device called a small form factor pluggable (SFP) device is used, either as a discrete component between the cable and the FC-AL device or integrated within the cable assembly. FIGS. 1A and 1B illustrate versions of discrete SFP transceivers. Both integrated and discrete SFP transceivers contain a non-volatile memory component, such as a serial EEPROM, in which cable length information can be stored as cable vital product information (VPD). To access cable VPD information, several sideband signals were defined and implemented (I2C interface) by the SFP standards committee (see FIG. 2). The Mode Def bus incorporates an I2C interface; other pins are used to provide voltage to the SFP circuit board and its associated components.
Similar issues occur with serial attached SCSI (SAS) storage systems, However, the SAS external cable definitions do not provide any sideband signals for mini-SAS connectors (illustrated in FIGS. 3A, 3B and 4A, 4B) which may be used in future storage products. Indeed, only the high speed differential pairs (transmit and receive) and grounds have been defined; FIG. 5 illustrates the pin out list. As such, there is no current method to access VPD information, such as the cable length, if it were to be implemented within the cable assembly. Consequently, it remains desirable to provide cable length information for SAS external cable assemblies and to provide an interface mechanism that allows access to that information, while maintaining the SAS cable interface definition.